Major difficulties in regulating the self-oscillating Royer inverters for driving a CCFL primarily stem from the synchronization problem. Nowadays, lots of synchronization control strategies have been developed for solving the regulation problems on Royer inverters. The general approach to achieve the synchronization is only by modulating the converter center frequency with some degree of variations.
A typical current-fed Royer inverter for CCFL 1 is shown in FIG. 1. In which, the current source is formed by a buck converter 12. Switches S1 and S2 are in self-oscillation with a resonant tank CR and LR. The energy pump-up from the current-fed buck converter 12 is controlled by a PWM controller 11. Remarkably, the buck converter 12 may not pump enough energy for inverter in some control range because the center tap voltage Vx of the transformer T1 as well as the buck output voltage is always in a quasi-sinusoidal form. It may result in the uncertainty of conduction in the buck converter 12 due to the resonant quasi-sinusoidal voltage Vx could be higher than the input voltage Vdc. Besides, the way to regulate the Royer inverter should be to achieve the synchronization between the buck switch S3 and the resonant switches, S1 and S2, so as to increase the efficiency and reduce the EMI. The present invention presents a DTM-PWM controller to exactly achieve the synchronization of the self-oscillating Royer inverters. Appropriate analysis and experimentation are introduced to explore the DTM-PWM controller. The present invention provides an adequate start-up voltage and a quasi-sinusoidal current for driving the CCFL with relatively higher efficiency. This is relatively a simple and efficient approach to achieve a synchronous PWM controller for synchronizing the buck drives and the current-fed self-oscillating Royer inverter. The achievements of the present invention are verified with experiments.
Kept the drawbacks of the prior arts in mind, and employed experiments and research full-heartily and persistently, the dead-time-modulated synchronous PWM controller for the dimmable CCFL Royer inverter is finally conceived by the applicant.